Effects of Sn additions on the microstructure and impression creep behavior of AZ91 magnesium alloy

The microstructural evolution and creep behavior of a cast AZ91 alloy with 0.5, 1, and 2 wt% Sn additions were examined by impression tests in the temperature range 423-523 K under constant punching stresses in the range 150-650 MPa. The results showed that, for all loads and temperatures, the AZ91-2Sn alloy had the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the formation of Mg2Sn, reduction in the volume fraction of the eutectic β-Mg17Al12 phase, and solid solution hardening effects of Al and Sn in the Mg matrix. The respective stress exponents and activation energies of 5.0-6.9 and 110-150 kJ/mol were similar for all alloys studied. The decreasing trend of creep activation energy with stress suggests that two parallel mechanisms of lattice and pipe diffusion-controlled dislocation climb are competing. The former is the controlling mechanism at low stresses, while the latter prevails at high stresses.